1. Field of the Invention
The present invention relates to a radial gap type motor structure and more particularly to a radial gap type motor structure reduced in all of size, weight and cost.
2. Description of the Related Art
Recently, various office automation devices have been becoming higher and higher in performance, especially in operating speed. At the same time the devices have come to use a plurality of dedicated motors according to required functions. Particularly, in the case of small-sized DC motors, those enhanced in magnetic efficiency according to purposes of use, diminished in both cogging torque and electromagnetic noise and superior in controllability are desired. According to a measure adopted so far to meet this demand, the number of slots (salient poles) of a core and that of poles of a magnet are increased, whereby each magnetic circuit becomes short to decrease magnetic resistance, resulting in increase of magnetic efficiency. Further, as coil winding, there is adopted a concentrated winding to enhance the winding efficiency.
FIG. 11 shows the construction of a conventional laminated, inner rotor type brushless DC motor, and FIG. 12 is a sectional view thereof. In these figures, the numeral 1 denotes a lead wire and numeral 2 denotes a coil. A bobbin 9 is vertically fitted on each salient pole 10 of a stator 8 which formed by lamination of soft magnetic steel plates, and magnet wire is would thereon, to constitute the coil 2. Numeral 3 denotes a rotor magnet and numeral 4 denotes a sleeve which connects the rotor magnet 3 and a shaft 5 as a rotational center of a rotor with each other. Numeral 7 denotes a flange with a bearing 6 disposed centrally which bearing supports the rotor shaft 5. Two flanges 7 are formed in a sandwiching relation to the stator 8 from both sides. The tip of each salient pole 10 is formed wider than the winding portion to increase interlinkage flux with the rotor magnet 3 and hold the bobbin 9 (coil). The lamination length of the stator 8 is almost equal to the length of the rotor magnet 3 for the same reason as above.
However, as the number of slots increases, the slot spacing becomes narrower and hence the winding space is narrowed to the extent that a required volume of winding cannot be received in the winding space unless the motors size is increased. If the motor size remains the same, it is impossible to ensure a required volume of winding, so there is adopted a permanent magnet having high magnetic characteristics to obtain desired motor characteristics although this leads to an increase of cost. In the construction of the conventional lamination type stator, it is required that the lamination thickness of the stator 8 be made substantially equal to the length of the rotor magnet 3 in order to ensure a predetermined area of opposition between the salient poles 10 and the rotor magnet 3. As a result, the winding length for each turn of coil becomes larger and thus the winding efficiency is low. Besides, since the surface of each salient pole 10 opposed to the rotor magnet 3 is wide to ensure the aforesaid area of opposition and to hold the associated coil, so that the winding work efficiency is poor. This tendency is conspicuous particularly in the case of an inner rotor type motor. Further, the motor is heavy and difficult to handle because the stator is larger than necessary.